Raw or pig iron, used for making steel, is generally prepared by sintering a composition consisting of concentrated iron ore, consisting of iron oxide and residual silica (e.g., quartz), flux (e.g., limestone or dolomite), and coke. The flux is added to purge impurities, i.e., neutralize the silica. Slag, which results from this neutralization process, separates from the molten iron in the blast furnace. There is presently a desire to increase the efficiency of blast furnaces by reducing the volume of slag. Correspondingly, it is desired to reduce the cost of steel by reducing the amount of slag produced when processing pig iron, i.e., by reducing the amount of silica entering the blast furnace.
The concentrated iron ore used in steel making is obtained from mined ores comprised of "host rock", i.e., rock comprised of silica and iron oxide. In order to reduce shipping and handling costs iron oxide is normally separated from the host rock at or near the mine by, for example, magnetic and/or froth flotation processes. Many ores, in particular the taconite ore found in North America, require ultra fine grinding prior to carrying out such separation processes. Because of this grinding the concentrated ore is a fine powder which cannot be easily handled, shipped, or charged into a blast furnace.
Therefore, the concentrated ore is agglomerated into porous particles or pellets generally having an average diameter of approximately 3/8 inch by rotating the concentrated ore powder in a drum or disc with a binder and water to form balls, followed by firing the balls in an indurating furnace. The resulting particles or pellets are hard and are easily handled, shipped, and charged to a blast furnace.
Until recently, bentonite clay was the preferred binder for agglomerating concentrated ore particles; it provides moisture and growth control, and balls prepared with bentonite have very good wet and dry strength. But one problem with bentonite is that it contains a large amount of silica, which must be neutralized in the blast furnace, by adding additional amounts of flux. Accordingly, efforts have been made to replace bentonite with a product that has the beneficial attributes of bentonite without contributing silica. So called "organic based binders", such as alkali metal salts of carboxymethylcellulose, have been considered as replacements for bentonite; since they do not contain silica they do not require the use of additional flux, they burn off during sintering, and they increase the ratio of surface area to mass of the concentrated ore-containing pellet.
Also, the reduction of the pellets produced using organic binders is much more efficient because of their larger surface area. The rate of reduction of iron oxide to the metallic state in the blast furnace by reducing gases, such as carbon monoxide, is directly proportional to that surface to-mass ratio, in other words, to the pore volume. During sintering, bentonite fuses to a glass-like ceramic material that seals pores, while the pore volume of pellets produced using organic binders increases as the organic binder burns off.
Australian Patent Application No. AU-A 46544/85 describes a method for agglomerating a particulate material such as mineral ore concentrate comprising mixing the particulate material with a binding amount of water-soluble, ore binding polymer and clay. The polymer is hydroxyethyl cellulose in combination with sodium carbonate. U.S. Pat. Nos. 4,288,245 and 4,597,797 suggest that the binding Performance of alkali metal salts of carboxymethyl cellulose can be improved by inclusion of at least 2% of a salt of an alkali metal and a weak acid having a pK value higher than 3 and a molecular weight lower than 500, such as salts of acetic acid, benzoic acid, lactic acid, propionic acid, tartaric acid, succinic acid, citric acid, nitrous acid, boric acid and carbonic acid. One such binder is marketed under the name Peridur.sup.R and is believed to contain the sodium salt of carboxymethylcellulose, sodium carbonate, and byproducts from the formation of the carboxymethylcellulose, including sodium chloride and sodium glycolate.
The inventors have studied binder compositions of sodium carboxymethylhydroxyethylcellulose and sodium carbonate and discovered superior dry-strength performance versus binders known in the prior art.